The present invention relates to waveform synthesizers and, more particularly, to waveform synthesizers for producing electrical power outputs having waveforms of selected wave shape and/or frequency for driving and controlling AC machinery including induction, synchronous, and other type AC motors.
AC motors including single and poly-phase induction motors have found widespread application in industry where they are typically driven at a relatively fixed rotational speed by a constant frequency single or poly-phase power source. Various speed controllers for AC motors are known and have included speed controllers that synthesize a single or poly-phase waveform at a desired frequency by selectively switching oppositely poled DC power to the motor windings with the switching frequency controlling the motor speed. Known synthesizers take the form of analog control circuits, digital control circuits, or hybrid combinations of the two. Both analog and digital synthesizers usually require a large number of circuit components, although digital synthesizers may be fabricated at less cost because of the ease with which many digital components can be manufactured as integrated circuit devices.
One example of the many industrial and commercial applications of AC motors is their use in portable hand-held power tools. The AC motors used in hand-held power tools have usually been restricted to the constant speed, low torque type. Variable speed AC motors have not found widespread applications in portable power tools since known synthesizers for effecting speed control are either too costly from a marketing standpoint and/or too large for incorporation into the typical compact hand-held power tool. The use of a variable speed AC motor has certain advantages over the widely used commutator/brush-type motor in both the power tool and other variable speed applications in that the AC motor does not require the commutator/brush interface and, accordingly, can be thought of as a commutatorless motor. The elimination of the commutator/brush interface eliminates the problems of commutator and brush wear, electromagnetic interference generated as a consequence of commutation, and eliminates the brush sparking that occurs during commutation.
As can be readily appreciated, a physically compact and low-cost speed controller for the various types of AC motors including single and poly-phase motors would permit the wider application of these types of motors including their incorporation into variable speed power tools.